The present invention relates to a charge circuit, and, more particularly, to a charge circuit, a charge and discharge circuit and a battery pack that are suitable for a secondary battery used in portable electronic apparatus, such as notebook-size personal computers.
FIG. 1 is a schematic circuit diagram of a prior art charge and discharge circuit 100, which is conventionally built in a battery pack that is installed in a notebook-size personal computer. The function of the prior art circuit is described in the following to illustrate the limitations of conventional charge circuits. The charge and discharge circuit 100 controls charging and discharging of a battery 1 built in the battery pack. The battery 1 includes a plurality of (for example, three) lithium ion batteries 1a-1c connected in series. The charge and discharge circuit 100 includes a balanced current setting circuit 4 and a charging and discharging control circuit 7.
The positive electrode of the battery 1 is connected to a positive input/output terminal t1 via a discharge control switch 2 including a P-channel MOS (PMOS) transistor and a charge control switch 3 including the PMOS transistor. The negative electrode of the battery 1 is connected to a negative input/output terminal t2. When charges are charged into the battery 1, a direct current voltage is applied from a personal computer to the positive and negative input/output terminals t1, t2. When the charges of the battery 1 are discharged, driving power is supplied from the positive and negative input/output terminals t1, t2 to the personal computer.
When charges are charged into the battery 1, even if the discharge control switch 2 is turned off, a charge current is supplied from the personal computer to the battery 1 via the charge control switch 3 that is turned on and a parasitic diode D2 of the discharge control switch 2. When the charges of the battery 1 are discharged, even if the charge control switch 3 is turned off, a discharge current is supplied from the battery 1 to the personal computer via the discharge control switch 2 that is turned on and a parasitic diode D3 of the charge control switch 3.
The balanced current setting circuit 4 includes three charge current control circuits 4a-4c connected in parallel to the lithium ion batteries 1a-1c, respectively. The charge current control circuits 4a-4c include resistors 5a-5c and N-channel MOS (NMOS) transistors 6a-6c, respectively. When each of the NMOS transistors 6a-6c is turned on, charges of the corresponding lithium ion batteries 1a-1c are discharged.
The charging and discharging control circuit 7 includes a cell voltage detection circuit 8, an overcharge detection circuit 9 and an over discharge detection circuit 10. The cell voltage detection circuit 8 includes three cell voltage amplifiers 8a-8c that detect inter-terminal voltages (cell voltages) Va-Vc of the lithium ion batteries 1a-1c, respectively. Each of the cell voltage amplifiers is preferably an operational amplifier (op amp) having an amplification factor xe2x80x9c1xe2x80x9d.
Specifically, a non-inverting input terminal of the first cell voltage amplifier 8a is connected to the positive electrode of the first lithium ion battery 1a and the inverting input terminal thereof is connected to the negative electrode of the first lithium ion battery 1a (positive electrode of the second lithium ion battery 1b). The first cell voltage amplifier 8a detects the inter-terminal voltage (cell voltage) Va of the first lithium ion battery 1a and supplies the cell voltage Va to the overcharge detection circuit 9.
A non-inverting input terminal of the second cell voltage amplifier 8b is connected to the positive electrode of the second lithium ion battery 1b and the inverting input terminal thereof is connected to the negative electrode of the second lithium ion battery 1b (positive electrode of the third lithium ion battery 1c). The second cell voltage amplifier 8b detects the inter-terminal voltage (cell voltage) Vb of the second lithium ion battery 1b and supplies the cell voltage Vb to the overcharge detection circuit 9.
A non-inverting input terminal of the third cell voltage amplifier 8c is connected to the positive electrode of the third lithium ion battery 1c and the inverting input terminal thereof is connected to the negative electrode of the third lithium ion battery 1c. The third cell voltage amplifier 8c detects the inter-terminal voltage (cell voltage) Vc of the third lithium ion battery 1c and supplies the cell voltage Vc to the overcharge detection circuit 9.
The overcharge detection circuit 9 includes three comparators 9a-9c and an OR circuit 9d. The first cell voltage Va is supplied to a non-inverting input terminal of the first comparator 9a and a second reference voltage VTH is supplied to the non-inverting input terminal thereof. The first comparator 9a supplies a low-level detection signal to the OR circuit 9d when the first cell voltage Va is below the second reference voltage VTH. When the first cell voltage Va is equal to or greater than the second reference voltage VTH, a high-level detection signal is supplied to the OR circuit 9d. That is, the first comparator 9a supplies the high-level detection signal to the OR circuit 9d when the inter-terminal voltage Va of the first lithium ion battery 1a reaches the second reference voltage VTH.
The first comparator 9a has hysteresis and maintains the output of a high-level detection signal until the first cell voltage Va drops to a predetermined voltage VTL smaller than the second reference voltage VTH after the first cell voltage Va reaches a voltage equal to or greater than the second reference voltage VTH at one time.
The second cell voltage Vb is supplied to a non-inverting input terminal of the second comparator 9b and the second reference voltage VTH is supplied to the non-inverting input terminal thereof. The second comparator 9b supplies a low-level detection signal to the OR circuit 9d when the second cell voltage Vb is below the second reference voltage VTH. When the second cell voltage Vb is equal to or greater than the second reference voltage VTH, a high-level detection signal is supplied to the OR circuit 9d. That is, the second comparator 9b supplies the high-level detection signal when the inter-terminal voltage Vb of the second lithium ion battery 1b reaches the second reference voltage VTH.
The second comparator 9b has hysteresis and maintains the output of a high-level detection signal until the second cell voltage Vb drops to the predetermined voltage VTL smaller than the second reference voltage VTH after the second cell voltage Vb reaches a voltage equal to or greater than the second reference voltage VTH at one time.
The third cell voltage Vc is supplied to a non-inverting input terminal of the third comparator 9c and the second reference voltage VTH is supplied to the non-inverting input terminal thereof. The third comparator 9c supplies a low-level detection signal to the OR circuit 9d when the third cell voltage Vc is below the second reference voltage VTH. When the third cell voltage Vc is equal to or greater than the second reference voltage VTH, a high level detection signal is supplied to the OR circuit 9d. That is, the third comparator 9c supplies the high-level detection signal to the OR circuit 9d when the inter-terminal voltage Vc of the third lithium ion battery 1c reaches the second reference voltage VTH.
The third comparator 9a has hysteresis and maintains the output of a high-level detection signal until the third cell voltage Vc drops to the predetermined voltage VTL smaller than the second reference voltage VTH after the third cell voltage Vc reaches a voltage equal to or greater than the second reference voltage VTH at one time.
The output terminal of the OR circuit 9d is connected to the gate terminal of the charge control switch 3 (PMOS transistor). The OR circuit 9d turns off the charge control switch 3 when at least one of the detection signals of the comparators 9a-9c is at the high level. That is, when at least one inter-terminal voltage of the lithium ion batteries 1a-1c reaches the second reference voltage VTH, the charge control switch 3 is turned-off and charging is stopped.
The detection signals of the comparators 9a-9c are also supplied to gate terminals of NMOS transistors 6a-6c, respectively. For example, when the detection signal of the comparator 9a is at the high level, the NMOS transistor 6a is turned on and the charges of the first lithium ion battery 1a is discharged via the NMOS transistor 6a. Thus the inter-terminal voltage Va of the first lithium ion battery 1a drops to the predetermined voltage VTL. When the inter-terminal voltages Va-Vc of the lithium ion batteries 1a-1c drop to the voltage VTL, charging is restarted.
When the overcharge detection circuit 10 receives the inter-terminal voltages Va-Vc of each of the lithium batteries 1a-1c from the cell voltage amplifiers 8a-8c and at least one of the inter-terminal voltages Va-Vc drops below a predetermined third reference voltage, the charge control switch 2 is turned off and the discharging of the battery 1 is stopped. That is, when at least one of the inter-terminal voltages Va-Vc of the lithium ion batteries 1a-1c drops to a voltage equal to or smaller than the predetermined third reference voltage, the supply of driving power from the battery 1 to the personal computer is stopped.
Since there is dispersion in the lithium ion batteries 1a-1c, a difference arises in the inter-terminal voltages Va-Vc when charging starts or a difference arises at the time when the inter-terminal voltage reaches the second reference voltage VTH. In this case, the inter-terminal voltages Va-Vc of the lithium ion batteries 1a-1c will not reach the second reference voltage VTH at the same time. As a result, for example, when the first lithium ion battery 1a reaches the second reference voltage VTH, charging is temporarily stopped. After the inter-terminal voltage Va is dropped to the voltage VTL by discharging the charging charges of the first lithium ion battery 1a, the charging is restarted. The lithium ion batteries 1a-1c are evenly charged by repeating charging and discharging in this manner.
In the charging method of the aforementioned lithium ion batteries 1a-1c, however, a stress applies to each lithium ion battery and its life is shortened. Otherwise, since there is dispersion in the lithium ion battery, the charging and discharging are collectively repeated against a specific lithium ion battery and that lithium ion battery deteriorates faster than another lithium ion battery.
Japanese Patent Laid-Open Publication No. 7-87673A discloses a charging control system that charges a battery without repeating charging and discharging. The charging control system comprises a plurality of detectors that detect the inter-terminal voltages of a plurality of lithium ion batteries and a plurality of bypass circuits connected in parallel to the plurality of lithium ion batteries, respectively. When the inter-terminal voltages of the lithium ion batteries detected by the detectors reach a predetermined voltage, the associated bypass circuit is conducted and charging is terminated.
In the aforementioned charging control system, however, regardless of the dispersion of the inter-terminal voltage of each lithium ion battery, the bypass circuit associated with the battery that reached the predetermined voltage is conducted. Therefore, if the dispersion of the inter-terminal voltage is large, the bypass circuit conducted first is kept in the conduction state for a long time. As a result, the durability of the bypass circuit decreases and the power consumption increases.
Japanese Patent Laid Open Publication No. 7-87673A discloses a charging control system that performs charging in the state where the inter-terminal voltages of two lithium ion batteries are always equal. The charging control system includes a resistance voltage dividing circuit that generates half the voltage of the total voltages of two batteries and an amplifier that amplifies the half voltage at an amplification factor xe2x80x9c1xe2x80x9d. The output of the amplifier is connected to the node of the two lithium ion batteries. When the inter-terminal voltage of the lithium ion battery on the positive electrode side is greater than the inter-terminal voltage of the lithium ion battery on the negative electrode side, the lithium ion battery on the negative electrode side is charged by the amplifier. When the inter-terminal voltage of the lithium ion battery on the positive electrode side is smaller than the inter-terminal voltage of the lithium ion battery On the negative electrode side, the lithium ion battery on the positive side is charged.
In the aforementioned control system, however, even when the two lithium ion batteries are discharged, the amplifier and the voltage dividing circuit are activated by the power supply voltages of the batteries, so that the power consumption increases. As a result, the service time of a battery pack is shortened. Moreover, applying the aforementioned control system to three lithium ion batteries or more makes the circuit configuration very complicate.
An object of the present invention is to provide a charge and discharge circuit that reduces the power consumption at charging and discharging without shortening the life of a secondary battery.
In a first aspect of the present invention, a charge circuit that charges a plurality of secondary batteries is described. The charge circuit includes a plurality of charge current control circuits connected in parallel to the plurality of secondary batteries for performing bypass control of a charge current supplied to the plurality of secondary batteries. A potential difference detection circuit is connected to the plurality of charge current control circuits to detect a voltage difference between the plurality of secondary batteries and control the plurality of charge current control circuits in accordance with the voltage difference to selectively bypass the charge current supplied to the plurality of secondary batteries.
In a second aspect of the present invention, a charge circuit that charges a plurality of secondary batteries is provided. The charge circuit includes a plurality of charge current control circuits, connected in parallel to the plurality of secondary batteries, respectively, for performing bypass control of a charge current supplied to the plurality of secondary batteries. A plurality of potential difference detection circuits are connected to the plurality of charge current control circuits, respectively. Each potential difference detection circuit detects a voltage difference between a voltage of the associated secondary battery and a voltage of another secondary battery and controls the associated charge current control circuit in accordance with the voltage difference to selectively bypass the charge current supplied to the associated secondary battery. A charge detection circuit is connected to the plurality of potential difference detection circuits to detect whether charging is performed into the plurality of secondary batteries and activate the plurality of potential difference detection circuits when the charging is performed.
In a third aspect of the present invention, a charge and discharge circuit that performs the charging and discharging of a plurality of secondary batteries is provided. The charge and discharge circuit includes a plurality of charge current control circuits, connected in parallel to the plurality of secondary batteries, for performing bypass control of a charge current supplied to the plurality of secondary batteries. A potential difference detection circuit is connected to the plurality of charge current control circuits to detect a voltage difference of the plurality of secondary batteries and control the plurality of charge current control circuits in accordance with the voltage difference to selectively bypass the charge current supplied to the plurality of secondary batteries. A charge detection circuit is connected to the potential detection circuit to detect whether charging is performed into the plurality of secondary batteries and activate the potential difference detection circuit when the charging is performed. An overcharge detection circuit detects the voltages of the plurality of secondary batteries and stops the charging into the plurality of secondary batteries when and at least one of the detected voltages is equal to or greater than a predetermined first reference voltage. An over discharge detection circuit detects the voltages of the plurality of secondary batteries and stops discharging of the plurality batteries when at least one of the detected voltages drops to a voltage equal to or smaller than a predetermined second reference voltage.
In a fourth aspect of the present invention, a charge and discharge circuit that performs the charging and discharging of a plurality of secondary batteries is provided. The charge and discharge circuit includes a plurality of charge current control circuits, connected in parallel to the plurality of secondary batteries, respectively, for performing bypass control of a charge current supplied to the plurality of secondary batteries. A plurality of potential detection circuits are connected to the plurality of charge current control circuits, respectively. Each potential detection circuit detects a voltage difference between the voltage of associated secondary battery and the voltage of another secondary battery and controls the associated charge current control circuit in accordance with the voltage difference to selectively bypass the charge current supplied to the associated secondary battery. A charge detection circuit is connected to the plurality of potential difference detection circuit to detect whether charging is performed into the plurality of secondary batteries and activate the plurality of potential difference detection circuits. An overcharge detection circuit detects the voltages of the plurality of secondary batteries and stops the discharging of the plurality of secondary batteries when at least one of the detected voltages is equal to or greater than a predetermined first reference voltage. An over discharge detection circuit detects the voltages of the plurality of secondary batteries and stops the discharging of the plurality of secondary batteries when and at least one of the detected voltages drops to a voltage equal to or smaller than a predetermined second reference voltage.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.